Platelet activation at sites of vascular injury begins when a locally- generated agonist such as thrombin or collagen interacts with receptors on the platelet surface. This initiates a signal within the platelet that is propagated by a succession of molecules that includes G proteins, phospholipases, various second messengers including Ca++, and a variety of serine/threonine- and tyrosine-specific protein kinases. Ultimately, this results in platelet shape change, the exposure of fibrinogen receptors on the platelet surface, granule secretion and aggregation. Arguably, the most biologically-significant trigger for these events in vivo is thrombin. Recent studies in this laboratory have concentrated on the regulation of Ca++ homeostasis in platelets, the role of IP(3), the structure and function of platelet G proteins, the mechanisms of fibrinogen receptor expression and, most recently, the structure and function of platelet receptors for thrombin. The studies proposed for the next 5 years represent a natural evolution of several of these themes. The major focus will be upon the platelet receptor, including the manner in which it interacts with G proteins and the mechanisms by which its activity is initiated and terminated. By extension these results are applicable to other platelet receptors for biologically significant agonists, and these will also be examined. Our specific goals are to: (1) Define the mechanisms of platelet activation by thrombin. (2) Identify the molecular basis for signal termination and receptor down-regulation following platelet activation by thrombin. (3) Define the role of G proteins in platelet activation, particularly those G proteins that interact with thrombin receptors in platelets and endothelial cells. (4) Apply the approaches used in studying thrombin receptors to other platelet receptors, including the thromboxane A(2) receptor.